As shown in FIG. 12, a viscous damper for absorbing a torsional vibration of a crankshaft of an internal combustion engine includes a case 3 with an opening 2a provided at one side in an axial direction of an annular case body 2 substantially U-shaped in section being closed with a cover 7 to be liquid-sealed, and an annular inertia mass element 4 housed with a damping liquid (not shown) inside the case 3, and the viscous damper is fixed with a bolt to a revolving shaft 1 of the internal combustion engine via a mounting portion 2c integrally formed at the case body 2.
Inside the case 3, a closed chamber 2b is defined by the case body 2 and the cover 7, the inertia mass element 4 is placed rotatably in a circumferential direction in the closed chamber 2b with its both sides in an axial direction being supported by thrust bearings 5, and with an inner circumferential surface of the inertia mass element 4 being supported by a journal bearing 6.
The thrust bearing 5 is constituted of a disc of a material with lubricity such as a nylon resin, and is mounted by being fitted in a shallow mounting hole 8 formed on both side surfaces in a diameter direction of the inertia mass element 4. The thrust bearings 5 abut to an inner surface of an orthogonal wall at an opposite side from an open side of the case body 2 and an inner surface of the cover 7, and rotatably support the inertia mass body 4 in a circumferential direction with a fixed narrow clearance being kept between these inner surfaces. The journal bearing 6 is constituted of an annular ring of a nylon resin or the like, and formed to be an open ring cut at one spot. The journal bearing 6 is placed between an inner circumferential surface of the inertia mass element 4 and an inner surface of an inner circumferential wall of the case body 2, and supports the inertia mass element 4 rotatably in the circumferential direction with a fixed narrow clearance being kept from the inner surface (refer to, for example, Japanese Utility Model No. 2579119).
In order to obtain a desired damping characteristic, this viscous damper needs to adjust the inertia mass element 4 correspondingly to the characteristic of the internal combustion engine to which the viscous damper is applied, and if the inertia mass element 4 is changed, the case body 2 and the cover 7 need to be changed. The inertia mass element 4 is generally formed by casting due to less waste of the material and its suitability for mass production, and if the inertia mass element is changed, the casting die has to be changed.
The damping force of a viscous damper is derived from occurrence of a shear resistance force to the damping liquid with high viscosity existing between the case 3 and the inertia mass element 4 when a relative speed difference between the case 3 and the inertia mass element 4 occurs, and therefore the damping force is influenced not only by the dimension of the clearance between the case 3 and the inertia mass element 4 but also by the conditions of the inner surface of the case 3 and the outer surface of the inertia mass element 4 to a large extent. Consequently, the cast inertia mass element needs finishing work on its surface by cutting or the like, and therefore the production process is complicated.
For this reason, the development of a viscous damper with excellent general versatility, easy to produce and capable of reduction in the production cost is desired.
Accordingly, an object of the present invention is to provide a viscous damper improved in general versatility, easy to produce, and capable of reduction in the production cost.